The class of the Arachnida may be divided into the following orders:
Respiration effected by the general surface of the body; limbs four pairs in number, elongated; abdomen rudimentary, unsegmented; sexes distinct.
The members of this order, sometimes called "Sea-spiders," have been placed alternately amongst the Arachnida and the Crustacea, their true position being rendered doubtful by the fact that, though marine in their habits, they possess no differentiated respiratory organs. They possess, however, no more than four pairs of legs, and would therefore appear to be properly referable to the Arachnida. According to Dr Dohrn, however, the embryo is naupliiform, and this would support a reference of the order to the Crustacea. The commoner forms of the Podosomata (such as Nymphon and Pycnogonum) may be found on the sea-coast at low water, crawling about amongst marine plants or hiding beneath stones. Some species of the latter genus are parasitic upon fishes and other marine animals, but the common British species (P. littorale) is free when adult, and does not appear to be parasitic at any stage of its existence (fig. 164, a). The legs consist of four pairs, sometimes greatly exceeding the body in length, and containing caecal prolongations of the digestive cavity for a portion of their length. The mouth is sometimes provided with a pair of "chelicerae," or chelate mandibles, and with two well-developed maxillary palpi, behind which in the female is a pair of false legs which carry the ova. The abdomen is rudimentary; but the cephalothorax is segmented. Though there are no respiratory organs, there is a distinct heart. The sexes are in different individuals, and the larvae have at first only two pairs of legs.